1. Field of the Invention
The present invention relates to a pump unit and a hydrostatic transmission.
2. Related Art
It has been conventionally known that, in pump units configured to be capable of transmitting a rotational power from a driving power source that is operatively connected thereto to an actuator through a pair of hydraulic fluid lines, and in hydrostatic transmissions (hereinafter, also referred to HST) which non-stepwisely change the speed of a rotational power from a driving power source which is operatively connected thereto by using the hydraulic pressure of hydraulic fluid flowing through a pair of hydraulic fluid lines, there is provided a PTO transmission mechanism for branching and outputting the power from the driving power source.
In pump units or hydrostatic transmissions as described above, there may be provided an auxiliary pump main body which is operatively driven by the driving power source in order to supply hydraulic fluid for replenishing to the pair of hydraulic fluid lines and supply hydraulic fluid for operating to the PTO clutch mechanism provided in the PTO transmission mechanism (refer to, for example, JP-A 2003-291674).
A conventional pump unit or conventional hydrostatic transmission includes a suction line for fluidly communicating between a suction port of the auxiliary pump main body and a fluid reservoir, a discharge line which is fluidly connected to a discharge port of the auxiliary pump main body, a charge line which is branched from the discharge line via a pressure-reducing valve for supplying hydraulic fluid for replenishing to the pair of hydraulic fluid lines, and a PTO hydraulic fluid line which is branched from the discharge line via the pressure-reducing valve for supplying hydraulic fluid for operating to the aforementioned PTO clutch mechanism.
In the conventional pump unit or the hydrostatic transmission, the charge line is fluidly connected to a secondary side of the pressure-reducing valve and the PTO hydraulic fluid line is fluidly connected to a spring chamber of the pressure-reducing valve. Consequently, when the PTO clutch mechanism is engaged or disengaged, the hydraulic pressure in the charge line is fluctuated, thus causing the malfunction of fluctuations in the operation torque of the output-adjusting member used for changing and operating the suction/discharge rate of the hydraulic pump main body.
Namely, the pressure-reducing valve is configured to be capable of defining the hydraulic pressure in the charge line to a predetermined hydraulic pressure with a hydraulic pressure setting spring placed within the spring chamber and, as previously described, a conventional pump unit or hydrostatic transmission is configured such that the PTO hydraulic fluid line is supplied with hydraulic fluid from the spring chamber of the pressure-reducing valve.
In the PTO hydraulic fluid line, there is inserted a PTO relief valve for setting the hydraulic pressure in the PTO clutch mechanism, in addition to a switching valve for switching between supply and interruption of hydraulic fluid to the PTO clutch mechanism. Therefore, when the PTO clutch mechanism is disengaged, the PTO hydraulic fluid line is at a natural pressure that is communicated with the fluid reservoir, while when the PTO clutch mechanism is engaged, it is set to an operational pressure defined by the PTO relief valve.
Consequently, within the spring chamber of the pressure-reducing valve connected to the PTO hydraulic fluid line, the hydraulic pressure is varied along with the hydraulic pressure fluctuation in the PTO hydraulic fluid line. Thus, the hydraulic pressure in the charge line, which is defined by the hydraulic pressure setting spring within the spring chamber, is also varied along with the hydraulic pressure fluctuation in the PTO hydraulic fluid line.
As described above, with conventional pump units and hydrostatic transmissions, there have been caused malfunctions of fluctuation in the operation torque of the output-adjusting member of the hydraulic pump main body due to the difference of the hydraulic pressure in the charge line between when the PTO clutch mechanism is engaged and when it is disengaged.
Also, in order to gradually raise the hydraulic pressure in the PTO hydraulic fluid line to the hydraulic pressure defined by the PTO relief valve for preventing abrupt engaging movement of the PTO clutch mechanism, an accumulator may be inserted in the PTO hydraulic fluid line, in some cases.
However, sufficient considerations have not been given to the placement of such an accumulator in conventional pump units and hydrostatic transmissions.
Also, pump units as described above may include a pair of hydraulic pump main bodies. Such pump units are configured to be capable of transmitting a rotational power from a driving power source which is operatively connected thereto to first and second hydraulic motor units for driving the left and right driving wheels through a pair of first hydraulic fluid lines and a pair of second hydraulic fluid lines.
Namely, in the pump unit, the pair of hydraulic pump main bodies and the pair of hydraulic motor units form a pair of HSTs for individually driving the pair of left and right driving wheels.
However, with a conventional pump unit including such a pair of hydraulic pump main bodies, even though the respective output-adjusting members in the pair of HSTs are operated such that the pair of hydraulic motor units are brought into the same output state, there may occur output differences between the pair of hydraulic motor units due to fabrication errors and assembly errors.
Such output differences between the pair of hydraulic motor units may cause a so-called wandering phenomenon that the vehicle meanders even though the vehicle is manipulated such that it linearly moves.
The present invention was made in view of the aforementioned prior art. It is an object of the present invention to provide pump units or hydrostatic transmissions including a PTO clutch mechanism, the pump units or hydrostatic transmission being capable of maintaining the charge pressure for a pair of hydraulic fluid lines constant, regardless of whether the PTO clutch mechanism is engaged or disengaged.
It is another object of the present invention to provide pump units or hydrostatic transmissions including an accumulator for gradually raising the hydraulic pressure of hydraulic fluid for the PTO clutch mechanism, the pump units or hydrostatic transmissions being capable of preventing the increase of the apparatus size and also enhancing a piping workability for the accumulator.
It is still another object of the present invention to provide a pump unit for vehicle-traveling configured to include a pair of hydraulic pump main bodies which are operatively connected to a driving power source and transmit a rotational power from the driving power source to first and second hydraulic motor units for driving right and left driving wheels, through a pair of first hydraulic fluid lines and a pair of second hydraulic fluid lines, the pump units being capable of individually driving the right and left driving wheels in a variable-speed manner and effectively preventing the occurrence of wandering phenomena during straight-movement operation of the vehicle.